And just so we’re clear: the latest possible date for the DSS is now usually given as AD 70. Generally, the scrolls are assigned a 200 BC-AD 70 range, with a small portion stretching back to the 3rd century BC, and the bulk of the extant material dating from the 1st century BC.
That’s all that I said, and all that I meant: that some of the DSS come from years after Christ. If I’m not mistaken, the Qumran community, and the DSS, showed spikes in activity every so often, and the second to last of those was during the First Jewish Revolt AD 66-70, and the last was in the Third Jewish Revolt/The Bar Kochba Revolt in AD 132-135, after which the community went silent for all time (this is where my “Bar Kochba snark” came from).
I never made a specific commitment to a date for 1QIsa, but anything earlier than BC 70 seems to stretch the evidence to me. BC 100 is close to the edge of reason.
Also, no matter the percentages (which you just pulled off the Wikipedia article, and which I put there!), the fact that any number of scrolls that can not be accounted for by chance parallel the LXX demonstrates that the LXX is an exemplar of a most ancient textual tradition, in the face of some who would like to claim it was still be altered at Jerome’s time (confusing the Old Greek with Theodotion’s Daniel and Symmachus).
The matter isn’t that the majority of textual witnesses agree with the LXX - it’s that the most ancient ones do. The older a specific fragment, the more likely it’ll be closer to the LXX reading.
If so, analysts a millennium in the future would have to conclude from our Bibles in the West that the Septuagint was completely lost somewhere between 450 and 1500 AD, as we don’t use it; the greater the time differential between us and the original, the more we tend to lean towards the Hebrew, for some reason, our scholars considering it to be more “original”, as it was the ancestral tongue (albeit forgotten after the exile) of the people who originally received the revelation; only later was it revived (the Hebrew has shown a great resilience, unlike Latin, in becoming either a comatose or dead language and then reviving). However, before the DSS were discovered, it (the LXX) was demonstrably less original and more corrupt than the tradition preserved by the LXX (and even Theodotion); after the discovery of the DSS, it’s about equal to most of them, some DSS being more ancient, some being less.
And there’s no doubt the LXX was made over a decently long period of time, beginning somewhere around BC 300-250 for the Torah, and likely ending in the middle of the second century BC for the Prophets and Writings. If one wants to consider the books of Maccabees and Sirach as having been long canonical and based on Hebrew originals, the translation may have ended in the early first century BC - around the same time as 1QIsa.
Also, the skill and style of the translators varied greatly. The Torah (all translated at once, probably the root, if any, of the old “Legend of 70” and the name “LXX”) is beyond literal, almost an interlinear translation, which shows that a strong Hebrew underlay it, and possibly, that it was used in conjunction with the Hebrew for a time (in which cases, it would always reflect the current Jewish understanding of the text). However, that textual tradition is long dead and lost, except for the exemplars rediscovered in the last 60 years in Qumran. Isaiah, being the book where the rendering “virgin” comes from, denotes either a textual variant to “almah” (probably “bethulah”), or the current understanding at the time of Isaiah’s translation of the passage, being something like “a young woman who is a virgin” (we can see a similar thing today looking at the book, wondering, “Why is the LORD calling a young woman conceiving a sign? It’s not miraculous.”). The translator of Isaiah is also idiosyncratic; he travels between the greatest extremes of completely flawless eloquence to the depths of near incompetence many times - this, the last sentence, being the best argument against the interpretation of “virgin”.
I just saw your graph (no, I didn’t make that one, someone else did): is it any coincidence that the MT has no identifiable ancestors - i.e., they’re all lost - until it pops up? Every other textual tradition has at least limited provenance. The main question this stands for is in pre-DSS times, the MT was demonstrably inferior; today, it’s got some bolstering (as has the LXX) from the DSS - only in the last 60 years has it become a historically and intellectually viable alternative to the LXX. So why the Hell did translators use it 400 years ago? (Albeit old translations - KJV and earlier - generally were much more liberally sprinkled with LXX readings than modern ones, the NRSV finally reversing the trend since the RV1881 of “more Hebrew!!!”, albeit at too great of a cost - pervasive feminism - for me to bear. On the other hand, Catholic translations until Divino Afflante Spiritu were all based on the Vulgate, itself based on ancient and lost Hebrew, another witness to the “virgin” reading [unless you’d argue that the Vulgate’s Isaiah is the VL with minor emendation].)